The present invention relates to an antenna positioning method and an antenna mounting device for a communication device, and an antenna device.
Communication devices having a transmission diversity functions, receive diversity functions and MIMO (Multiple Input Multiple Output) communication functions, for performing communication using a plurality of antennas, are available. FIG. 11 is a diagram depicting a receive diversity, showing a space diversity configuration comprised of one transmission antenna ATt and two receive antennas ATr0 and ATr1. If the channel response characteristics between the transmission antenna ATt and each of the two receive antennas ATr0 and ATr1 are h0 and h1, noise between the transmission antenna ATt and each of the receive antennas ATr0 and ATr1 are n0 and n1, the transmission signal is x0 and the receive signals of each of the receive antennas ATr0 and ATr1 are r0 and r1, then the following expressions are established.r0=h0x0+n0  (1a)r1=h1x0+n1  (1b)
FIG. 12 is a diagram depicting a maximum ratio combination, where the channel estimation units 1a0 and 1a1 in branches #0 and #1 (receive antennas ATr0 and ATr1) estimate the channel response characteristics h0 and h1 in the branches #0 and #1 and output the complex conjugate h0* and h1* of the channel response characteristics respectively, and channel compensation units 1b0 and 1b1 multiply the receive signals r0 and r1 by h0* and h1* respectively, and outputs0=h0*r0  (2a)s0=h1*r1  (2b)or output the following respectively.s0=|h0|2x0+h0*n0  (2a)′s0=|h1|2x0+h1*n1  (2b)′
A space diversity combining unit 1c combines the output of the channel compensation unit 1b0 and 1b1 of each branch, and inputs the following combined signal to a demodulation unit 1d. s0+s1=(|h0|2+|h1|2)x0+h0*n0+h1*n1  (2c)In the maximum ratio combining method,(|h0|2+|h1|2)becomes a diversity gain.
In such a communication method using a plurality of antennas, it is necessary to decrease correlation, ideally to none, between the antenna receive signals. In FIG. 11, for example, if the correlation of the receive signals between the receive antennas ATr0 and ATr1 is 1, the receive state becomes worse when the channel response h0 (=h1) becomes worse, which makes diversity reception meaningless. But if the correlation is 0, then even if one channel response h0 becomes worse, the other channel response h1 does not become worse, and a good receive state can be maintained.
In a case of a communication device which can place a sufficient distance between antennas, the phase characteristics of the receive signal of each antenna change because of the distance difference, and correlation of the receive signals decreases. But in a case of a communication device of which antenna mounting positions are limited, such as a portable telephone, the distance between antennas is not sufficient, and correlation between the antenna receive signals becomes high, and as a result, the diversity function cannot be exerted as much as the case of no correlation, and the radio characteristics, including the error rate, deteriorate.
A conventional technology on radiation characteristics of an antenna is directional diversity that increases radiation characteristics of an antenna in a direction at which the radiation characteristics of another antenna is low, so that the radiation characteristics of the antennas compensate each other (Japanese Patent Application Laid-Open No. H7-143102), which can decrease correlation. In other words, according to this prior art, the radiation patterns of the two antennas are designed so as to be like petals in a conjugate relationship, as shown in (a) and (b) of FIG. 13, and one having a greater direct wave or indirect wave is selected.
However it is difficult to design antennas which have the radiation patterns in FIG. 13. Also the prior art does not fully consider the relationship of the radio incoming direction or the radio radiation direction and the antenna positions, and therefore the radiation pattern of each antenna is not always used efficiently.